The disclosed invention relates to metrological instruments. In particular, it relates to a method and apparatus for the robust estimation of tilt and offset of planes in a two-dimensional map. More specifically, in the preferred embodiment, the invention relates to the robust estimation of tilt and offset of planes in phase profilometer height mapping techniques.
The printed-circuit board industry has grown increasingly automated in the past few years. The use of surface-mount technology (SMT) has become dominant. In this technology, a paste of solder and flux is applied to the bare circuit board by means of metal stencil and rubber squeegee. The stencil is removed, leaving solder paste xe2x80x9cbricksxe2x80x9d in appropriate places. Components are then placed on the solder bricks by machine. The solder paste has sufficient viscosity and surface tension to hold the components to the circuit boards temporarily. The stuffed boards are then passed through an oven, which melts the solder paste and makes secure mechanical and electrical connection.
Another technology, called xe2x80x9cflip-chipxe2x80x9d technology, affixes solder balls directly to an integrated-circuit chip. The chip is inverted (hence the name xe2x80x9cflip chipxe2x80x9d) and placed directly onto the circuit board. The board is passed through an oven to melt the solder and make the electrical and mechanical connection.
Along with this automation, the need has arisen for automatic inspection of the circuit boards prior to the placement of components on them. Defects in the stencil or squeegee can result in uneven or missing solder bricks, which can cause a component lead to fail to make contact with the circuit board. The earlier this defect can be caught, the less money and time will be wasted on further processing and testing of a defective board.
U.S. patent application Ser. No. 08/820,380, titled Method and Apparatus For Three Dimensional Imaging Using Multi-Phased Structured Light, assigned to the assignee of this application and now U.S. Pat. No. 6,049,384, discloses an instrument for the automatic inspection of the solder paste pads of a circuit board prior to the placement of components onto the board. Copending U.S. patent application Ser. No. 08/607,845, titled Method And Apparatus For High Precision Three Dimensional Imaging Using Multi-Phased Structured Light, discloses an instrument useful for the inspection of the solder balls on the flip chip prior to placement on the circuit board. Both applications are herein incorporated by reference.
The instruments disclosed in the above-referenced copending applications both assess the surface topography of a target object by producing an image from the reflection of structured light projected onto the surface of the target object. The intensity of the detected light corresponds to the height of the target object surface. The imaging method used by the instruments suppresses extraneous artifacts due to traces, silk-screened lettering, etc., and clearly reveals the solder, since it has a height dimension that the extraneous artifacts do not have.
A critical factor in the processing of these images is a robust means of measuring the heights of the solder bricks or balls. Since the location and registration of the circuit board or flip chip cannot be precisely controlled, the height and tilt of the substrate are subject to variations. Further, since the important measurement is the height of the solder above the substrate, a simple measurement of the absolute height of the solder will be affected by the errors in board registration. It is therefore necessary to estimate the height of the substrate at each solder brick or ball location, so that a differential measurement can be made of the solder heights. Generally, for the preferred methods and apparatus of this invention, this operation can be considered to be the estimation and subtraction of a tilted reference plane from the height map.
The measurement of the substrate height is relatively straightforward in principle, but there are a number of complications in practice which must be overcome.
1. The substrate may have tilts as well as offsets. The tilts need to be estimated.
2. Circuit boards usually contain a number of parallel planes, e.g., bare board, solder mask, solder pads, etc. Each of these planes lies at a different height. Depending on the proportion of each type of surface feature, the average height in a given field of view is apt to vary. Thus a simple average is unsatisfactory. A least-squares fit to the substrate is apt to give particularly unsatisfactory results if one side of the image contains a higher proportion of tall planes, and the other side of the image contains a lower proportion of tall planes.
3. The very target objects to be measured, the solder bricks and balls, will have a large effect on the average height in a given field of view; the reference plane estimate must not be affected to any great degree by them. Since the purpose of this substrate de-tilting is to make the feature extraction and measurement easy, attempts to locate the balls and exclude them from reference-plane calculations lead to a dilemma.
4. The height map may be in the form of a phase image, whose circular nature gives rise to the phenomenon of phase wrapping. Before an appropriate offset is applied, phase wraps that may exist in the image must be considered. An effective estimation technique must not be affected by this phenomenon.
Prior art practice generally involved measurement of the brick or ball heights by measuring the height of the substrate somewhere near the brick or ball. This technique is subject to excessive error, since if a tilt exists in the substrate, the height of the substrate underneath the brick or ball is not the same as the height of the substrate in the location where it is measured. Furthermore, owing to irregular circuit-board topography, even if the substrate were not tilted, the substrate height itself may vary.
Accordingly there is a need for a method and device which can provide accurate and reliable measurement of background tilt and offset of planes in a two-dimensional map of a reference substrate while overcoming the limitations and problems of the prior art.
The problems outlined above are in large measure solved by the method and apparatus for estimating background tilt and offset in accordance with the present invention. The invention is particularly useful in conjunction with high-speed image-processing operations for quality-control inspection on circuit-board production lines.
The method and apparatus in accordance with the present invention employs a process implemented in software, integrated with an optical measurement instrument. Specifically, the present invention calculates a merit function, which is greatest when the tilt of the reference plane coincides with the tilt of the substrate. A two-dimensional iterative search is used to maximize the merit function and thus establish the tilt of the reference plane. The two dimensions of the search are the degree of tilt about the x-and y-axes. This tilted reference plane can then be subtracted from the raw image, producing an untilted substrate. A further operation is then performed to estimate the height of the untilted substrate. This height offset can then be removed as well, which produces a height image where the heights of the features can be measured directly.
The choice of merit function is a central element of this technique. The invention employs a merit function that is calculated from a histogram of heights in the image. The basic concept recognizes that if the tilt in the substrate is removed, the parallel, flat planes in the substrate will each have constant height and will produce tall peaks in the histogram. This is true regardless of their distribution in the image. The example cited earlier, where a tall plane is on one side of the image, and a lower-lying plane is on the other side-of the image, is correctly handled by this histogram technique.
After the tilt of the reference plane has been established, its tilt can be removed from the height map, producing an untilted substrate. Further, the offset of the substrate can be removed to provide a realistic height map of the phase profile.